Forebrain monoamines and associative learning: III. The US pre-exposure effect.

This experiment evaluated the effects of forebrain norepinephrine (NE) and serotonin depletion on the unconditioned stimulus (US) pre-exposure phenomenon. Rats with either dorsal bundle lesions, raphe lesions, or which served as vehicle controls were assigned to one of two training conditions. One-half of the animals in the 3 lesion groups received 30 episodes of a stimulus which later served as the US. The remaining rats experienced standard conditioning of CS-US pairing without US pre-exposure. The results indicate that while raphe-lesioned and vehicular rats exhibited the US pre-exposure effect, conditioning was unimpaired in NE-depleted rats following pre-exposure. These findings are consistent with a contextual blocking interpretation of the US pre-exposure effect.